1. Field of the Invention
The present invention relates generally to color image reproduction systems, and more particularly to systems that improve color matching between original color images and reproductions of those images.
2. Description of the Related Art
Color image reproduction systems typically include an input device for providing a representation of an original image, an output device for generating a replica of the image, and a controlling device that processes signals received from the input device to generate new signals sent to the output device to produce the replica, which preferably is a high-fidelity reproduction of the original image. The controlling device may be implemented by a general-purpose computer with appropriate software and/or hardware for peripheral control and signal processing or may be a dedicated image processing unit associated with a particular input device/output device pair. Examples of an input device include hand held, flatbed and sheet-fed optical scanners, digital and video cameras, and software applications. In other words, an original image may be sensed or it may be created by a process. Examples of an output device include ink jet, laser, and photolithography printers, electrostatic, flatbed and drum plotters, and video displays such as cathode ray tubes, thin-film-transistor and liquid crystal display panels.
Generally, input and output devices use a device-dependent or device-specific color space or coordinate system to specify colors. These coordinate systems map the color coordinates to the color-sensing or color-generating process of the particular device. The term xe2x80x9ccolor spacexe2x80x9d refers to an N-dimensional space in which each point corresponds to a particular color.
One example of a three-dimensional color space is an RGB space in which point coordinates specify particular amounts of additive primaries red (R), green (G) and blue (B), represented by the three axes of the cube. All other colors within the cube can be represented by the R, G, B triplet, where the values for R, G, and B fall within a range from minimum intensity (e.g. 0) to maximum intensity (e.g. 1). White is represented when all three primaries are added at their maximum intensity (1,1,1) and black is represented by the complete absence (0,0,0) of all three primaries. Shades of gray are represented along the diagonal of the cube connecting the black and white coordinates. In an additive system light emitters are controlled to obtain color. The operation of many scanners and color display devices may be conveniently controlled by signals that are specified in RGB space. Another example of a three-dimensional color space is a CMY color space in which point coordinates specify particular amounts of subtractive primaries cyan (C), magenta (M), and yellow (Y) which combine to represent a specific color. In a subtractive system light is absorbed rather than emitted. Typically used in printing, the subtractive colors are printed on a white surface (e.g. paper) and the inks selectively absorb certain ranges of wavelengths of light and the remaining spectral radiant power is reflected. The subtractive system is the complement of the additive system and ideally the mixture of two additive primaries produces a subtractive primary. For example, the mixture of red and green is yellow. Similarly the mixture of two subtractive primaries produces a additive primary. When cyan and magenta are mixed, the cyan absorbs the red wavelengths of the magenta and the magenta absorbs the green wavelengths of the cyan, leaving only blue. Since complete absorption is difficult to achieve in printing inks, a fourth ink, black, is used as well in many devices. This system is specified as CMYK with the K representing black. The operation of many ink jet and laser printers may be conveniently controlled by signals that are specified in CMYK space or CMY space. Other color spaces that are related to particular devices are also known.
Most devices are capable of sensing or reproducing only a portion of the full range of colors that can be discerned by a human observer. A device xe2x80x9cgamutxe2x80x9d refers to the range of colors that can be sensed or reproduced by a particular device. For example, the gamut of a particular scanner refers to the range of colors that can be sensed by that scanner and the gamut of a particular printer refers to the range of colors that can be reproduced or printed by that printer.
A scanner gamut is determined by a variety of factors including the spectral response of the optical sensors, the spectral characteristics of color filters, spectral characteristics of the illumination source and the resolution and linearity of analogto-digital converters.
A printer gamut is determined by a variety of factors including spectral characteristics of colorants such as ink, spectral and porosity characteristics of media such as paper, resolution or dots-per-inch of the printed image, and half-toning methods applied.
A video display gamut is determined by a variety of factors including spectral characteristics of the light emitting material, type of display device, resolution of pixels or video lines, and excitation voltage.
Although it is possible in principle to construct a color image reproduction system by merely connecting an output device directly to an input device, the results generally would not be satisfactory because the device-dependent coordinate systems and color spaces for the input and output devices are generally not the same. Even if the two sets of coordinate systems and color spaces are the same, the fidelity of the reproduced image as compared to an original image would probably be very poor because the gamut of the input device generally is not coextensive with the gamut of the output device. Values representing xe2x80x9cout-of-gamutxe2x80x9d colors that are not in the output device gamut cannot be reproduced exactly. Instead, some xe2x80x9cin-gamutxe2x80x9d color that is in the gamut of the output device must be substituted for each out-of-gamut color.
Color image reproduction systems can achieve high-fidelity reproductions of original images by applying one or more transformations or mapping functions to convert point coordinates in one color space into appropriate point coordinates in another color space. These transformations may be conveniently performed by the controlling device. In particular, with respect to the output device gamut, transformations are used to convert values representing in-gamut and out-of-gamut colors in an input device-dependent color space into values representing in-gamut colors in an output device-dependent color space. The mapping of in-gamut colors and out-of-gamut colors is discussed separately.
Mapping In-Gamut Colors
The transformation of output device in-gamut colors for many devices are non-linear and cannot be easily expressed in some analytical or closed form; therefore, practical considerations make accurate implementations difficult to achieve. Many known methods implement these transformations as an interpolation of entries in a look-up table (LUT) derived by a process that essentially inverts relationships between measured responses to known input values. For example, a transformation for an input device may be derived by using a medium conveying patches of known color values in some device-independent color space such as the Commission International de L""Eclairage (CIE) 1931 XYZ space, scanning the medium with the input device to generate a set of corresponding values in some input device-dependent color space such as RGB color space, and constructing an input LUT comprising table entries that associate the known color XYZ values with the scanned RGB values. In subsequent scans of other images, scanned RGB values can be converted into device-independent XYZ values by finding entries in the input LUT having RGB values that are close to the scanned values and then interpolating between the associated XYZ values in those table entries. Various interpolation techniques such as trilinear, prism, pyramidal and tetrahedral interpolation may be used.
Similarly, a transformation for an output device may be derived by producing a medium with color patches in response to color values selected from some output device-dependent color space such as CMYK color space, determining the color value of the patches in a device-independent color space such as CIE XYZ space by measuring the patches using a spectral photometer, and constructing an output LUT comprising table entries that associate the measured color XYZ values with the corresponding CMYK values. In subsequent output operations, XYZ color values can be converted into device-dependent CMYK values by finding entries in the output LUT having XYZ values that are close to the desired values and then interpolating between associated CMYK values in those table entries. Various interpolations such as those mentioned above may be used.
In operation, a color image reproduction system scans an original image to obtained color values in an input device-dependent color space, transforms the scanned values into a device-independent color space, transforms these device-independent values from the device-independent color space into an output device-dependent color space and in response, generates a replica of the original image. As mentioned above, the transformations described thus far apply only to output device in-gamut colors.
Mapping Out-of-Gamut Colors
By definition, output device out-of-gamut colors cannot be reproduced exactly. Instead, high-quality color image reproduction systems use transforms or mapping functions that substitute an in-gamut color for each out-of-gamut color. Preferably, these transforms attempt to minimize the perceptible difference between each out-of-gamut color and the corresponding substitute in-gamut color. Techniques for transforming out-of-gamut colors into in-gamut colors generally map the out-of-gamut colors to the boundary of the output device gamut or compress a region of color space so that all desired colors are mapped into the output device gamut.
The process of deriving such a transform and generating the lookup table is called system color calibration. If the transform is derived for a particular scanner-printer combination, the system is referred to as a closed system and the process is called closed system color calibration, which is a special case of system color calibration.
Usually the color image reproduction system is calibrated in the factory and the transform for a particular scanner model and printer model is stored in a LUT. Once the system is in the field, however, the performance of the components may change over time, or there may be variations in performance from one printer to another even though they are the same model, or the properties of the print media may change. As a result, the system may need to be re-calibrated in the field.
When the system requires re-calibration, the printer is controlled to print out a pattern of color patches. This pattern is then scanned back into the system by the scanner. This is performed on site by a system operator. The pattern of color patches is usually an array of color blocks of various colors printed on a particular media. When this color patch sheet is placed on the platen of the scanner, several alignment problems may arise. The paper may be skewed or placed up-side down, for example. Also, if certain color patches are very light or white, the scanner may interpret such patch as the end of the array of color patches.
It is an object of the present invention to overcome the problems of media alignment and misidentification of color patches on a color patch sheet.
Another object of the invention is to improve the identification of color patch sheets used in a closed loop calibration system.
A further object of the invention is to provide specific identification marks on the color patch media to indicate pertinent information such as the media page number, dimensions of the color patch array, color patch size, etc.
The present invention addresses the problems of the prior art by providing a series of registration marks on the color patch sheet. These registration marks will be printed by the printer under control of the image processing unit and thereafter identified by a scanner under control of the image processing unit. For example, the registration marks may be small, black, rectangular blocks arranged at one or all of the top, bottom and sides of the page.
In a preferred embodiment the first mark at the top of the page represents the starting edge of the first color patch. The second mark is placed above the center or some pre-defined position of the first patch. From these two marks the width of the patches on the sheet can be calculated. The second mark can be placed at any selected position of the first mark, e.g. one-quarter or three-quarters width, as long as the total width of the patch can be calculated from the first and second marks. The next two marks are insurance marks that must follow the width marks (i.e. the first and second marks) in order for the image processing unit to recognize the width marks as legitimate (in contrast to stray marks on the page). The next mark can be one of a plurality of reserved marks that can be put to an assigned use such as the number of the sheet where the total of color patches being scanned comprise several sheets and or a print mode or print media identifier. The last two marks are end-width marks separated by the same distance as the first two width marks and that signal the end of the row of patches. This series of marks thereby provides the image processing unit with valuable information about the format of the color patch sheet being scanned, including the start, end and width of the patches. This prevents misinterpretation of color data on the sheet.
The location of the marks also provides information about the position of the color-patch sheet on the platen. Once the image processing unit finds each of the marks, it can determine the position of the sheet by the coordinates of the marks and correct for any skew using standard techniques such as bilinear transformation. The series of marks can be replicated at the side of the page to provide the information about the color patch array in the vertical direction, i.e. start, end and width of the patches in the vertical or longitudinal direction. Also, a series of marks are provided at the bottom of the page, which, because of their assigned pattern and spacing identify the location of the page bottom. This can signal the image processing unit that the sheet has been placed up-side down on the platen if this series of marks is encountered in the first horizontal scan.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.